Printed document comprising a luminescent authenticity feature

ABSTRACT

The invention concerns a printed valuable document with at least one authentication mark in the form of a luminescent substance based on a host lattice doped with at least one rare earth metal. The host lattice largely absorbs light in the entire visible region of the spectrum, is excitable in substantial parts of the visible region of the spectrum, and is at least partially transparent at least in the wavelength range between 0.8 μm and 1.1 μm. The host lattice also features a garnet or perovskite structure. The rare earth metal emits light in the wavelength range between 0.8 μm and 1.1 μm.

The invention concerns a printed valuable document with at least oneauthentication mark in the form of a luminescent substance based on ahost lattice doped with at least one rare earth metal.

The protection of valuable documents by means of luminescent substanceshas been known for a long time. The use of rare earth metals in thisconnection has also been discussed. These have the advantage ofpossessing, in the infra-red spectral region, narrow-band emissionlines, which are particularly characteristic, and can therefore besafely distinguished from the emissions of other substances when usingmeasuring technology. In order to increase protection againstcounterfeiting still further, the rare earth metals can be incorporatedtogether with other substances in host lattices, with the result thatthe excitation and/or emission spectrum of the rare earth metal isinfluenced in a characteristic manner. By combination with suitablyabsorptive substances, for example, a part of the excitation and/oremission bands of the rare earth metal can be suppressed. This influencecan, however, also take the form of a “distortion”, e.g. through dampingof specific regions of broad-band spectra.

Likewise for reasons of protection against counterfeiting, rare earthmetals with emission lines above 1500 nm are frequently used, sincedetection of the emission becomes more elaborate and more difficult thefurther into the IR spectral region the emission lines lie. For in verygeneral terms, the principle applies that the detection sensitivity ofphotodetectors decreases the longer the wavelength of the radiation tobe measured, since the signal-to-noise ratio becomes smaller in the sameproportion; i.e. the signals to be detected are more and more difficultto identify in the noise. The luminescent substances must therefore alsobe present in the valuable document in a certain minimum concentration,in order to be able to produce an adequate signal strength, which can bereliably detected in the noise. In certain instances, however, theselimit concentrations necessary for detection cannot be produced; forexample, if the luminescent substances have their own colour, whichdestroys the desired colour impression when mixed with the material ofthe valuable document. In some cases, the attempt is also made to reducethe risk of detection by chemical analysis of the luminescent substancescontained in a valuable document by the fact that the luminescentsubstances are applied only in very small concentrations.

In such cases, luminescent substances must be used for which theemission lines can be readily detected even in small concentrations.

The invention is therefore based on the aim of providing a valuabledocument with a luminescent substance producing emissions in the near IRspectral region so that, even at low concentrations, the presence of theluminescent substance can be demonstrated in the valuable document.

The fulfilment of this objective is provided by the non-dependentclaims. Further developments are the subject of the dependent claims.

The valuable document according to the invention contains at least oneluminescent substance based on host lattices doped with rare earthmetal, such that the rear earth metal emits in the near IR spectralregion, i.e. in the wavelength range of between 0.8 μm and 1.1 μm. Thisemission range has the advantage that its existence can be readilydetected with a silicon (Si), gallium arsenide (GaAs),gallium-indium-arsenide (Ga_(x)In_(x-1)As) or germanium (Ge)photodetector, since these have a relatively high response sensitivityin this wavelength region. As optically active rare earth metals,consideration may be given to the elements ytterbium, neodymium, orpraseodymium, or mixtures of these elements with one or more other rareearth metals.

These rare earth metals are embedded in a host lattice, which haseffective excitation bands in the visible region of the spectrum, andtransfers these excitation bands to the rare earth metals. The effectiveexcitation bands can be realised by, for example, chromium structuralelements which, according to the invention, are incorporated in a garnetor perovskite structure.

In this situation, the host lattice possesses an optical window in thenear infrared spectral region, and absorbs in virtually the entirevisible region of the spectrum, so that all the lines in the visiblespectral region of the luminescent substance are suppressed. Theexcitation range of the luminescent substances overlaps the radiationrange of strong light sources, such as halogen lamps, flash lamps, andsimilar sources. As a result of this, and due to the effective energytransfer to the rare earth metals within the host lattice, it ispossible to use very small quantities of substance with the valuabledocuments according to the invention, without the automatic detectioncapability being restricted. Detection by means of chemical analysis,however, is rendered extremely difficult due to the low concentration.

The absorptive constituents of the host lattice may in part be replacedby non-absorptive aluminium. The absorption, and therefore thebrightness of the luminescent substance, can be controlled through theproportion of aluminium. Luminescent substances of this type cantherefore also be used as additives for lighter printing inks.

Further embodiments and advantages of the invention are explainedhereinafter on the basis of the diagrams and the examples:

FIG. 1 Excitation spectrum of a chromium-containing lattice according tothe invention

FIG. 2 Spectra of several light sources

FIG. 3 Emission spectrum of a luminescent substance according to theinvention, doped with Pr

FIG. 4 Emission spectrum of a luminescent substance according to theinvention, doped with Nd

FIG. 5 Emission spectrum of a luminescent substance according to theinvention, doped with Yb

FIG. 6 Security element according to the invention, in a cross-sectionalview.

FIG. 1 shows the excitation spectrum of a chromium-containing latticeaccording to the invention. This lattice absorbs in almost the entirevisible spectral region. Due to this very broad-band absorption of thehost lattice, the lines produced in this region by the rare earth metaldopings are suppressed. At the same time, an energy transfer from thelattice to the rare earth doping takes place, thereby inducing theemission by the luminescent substance.

The broadband absorption of the lattice also has the advantage thatstrong light sources can be used for the excitation of the luminescentsubstances, such as flash lamps, which likewise emit radiation in theentire visible region of the spectrum.

FIG. 2 shows the spectrum of such a flash lamp with the referencecharacter 1. The spectrum 1 of the flash lamp shown extends continuouslyfrom the UV spectral region into the IR region. In some cases, it mayalso be a good idea to illuminate the luminescent substance only withlight from the visible spectrum range. In this case, illumination withlight-emitting diodes offers the appropriate wavelength. Light-emittingdiodes feature in general a narrow-band spectrum so that, to cover theentire spectral range, several LEDs are required. FIG. 2 shows thespectra 2, 3, 4 of a green, orange, and red light-emitting diode.

FIGS. 3, 4 and 5 show the emission spectra of individual luminescentsubstances according to the invention.

FIG. 3 shows the emission spectrum of a host lattice doped with Pr. Thespectrum extends from about 0.9 μm to about 1.08 μm. It features a verycharacteristic number of emission peaks, which can be assessed verysatisfactorily as authentication features.

FIG. 4 shows the characteristic spectrum of a lattice doped with Nd.This spectrum features two relatively strong emission peaks in the rangefrom about 0.9 μm to just under 1.1 μm. A somewhat smaller peak isadditionally located in the region of 0.95 μm.

The spectrum shown in FIG. 5 of a Yb-doped lattice is, by contrast, verysymmetrical and shows only one peak, the maximum of which is at 1.0 μm.

All these lattices according to the invention have the factor in commonthat they show a very striking luminescence emission in the nearinfrared region, i.e. in the range between 0.8 μm and 1.1 μm, which isdifficult to identify. Although all three emission spectra are arrangedin the same spectral region, they differ from one another sounambiguously that differentiation with measuring equipment is readilypossible.

In order to guarantee the highest possible effectiveness of the rareearth metals, in the case of a garnet structure, host lattices are usedwith the general formula:

A₃Cr_(5-x)Al_(x)O₁₂

where A stands for an element from the group of scandium, yttrium, thelanthanides, and the actinides, and the index x fulfils the condition0<x<4.99. Preferably, the index x moves in the range between 0.3 and2.5.

A preferred embodiment of the luminescent substance according to theinvention is:

Y_(3-z)D_(z)Cr_(5-x)Al_(x)O₁₂

where D stands for neodymium, praseodymium, or ytterbium, and the indexz fulfils the condition 0<z<1.

If the host lattice has a perovskite structure, it can be described bythe general formula:

ACrO₃

where A stands for an element from the group of yttrium, scandium, andthe lanthanides.

A preferred embodiment of the luminescence substance according to theinvention is a perovskite structure can be described by the followingformula:

Y_(1-z)D₂CrO₃

where D stands for one of the elements neodymium, praseodymium, orytterbium, and the index z fulfils the condition 0<z<1.

A number of examples of the luminescent substances according to theinvention are explained in greater detail below.

EXAMPLE 1

Manufacture of neodymium-activated yttrium-aluminium-chromium mixedgarnet (Y_(2.95)Nd_(0.05)Cr₄Al₁O₁₂):

47.82 g yttrium oxide (Y₂O₃), 7.32 g aluminium oxide (Al₂O₃), 43.65 gchromium (III) oxide (Cr₂O₃), 1.21 g neodymium oxide (Nd₂O₃), and 100 gdehydrated sodium sulphate (Na₂SO₄) are mixed intimately and heated to1100° C. in a corundum crucible for 12 hours.

After cooling, the reaction product is ground, the fluxing agent iswashed out with water, the sodium chromate produced as a side-product isreduced with sulphuric acid/iron (II) sulphate to chromium (III)sulphate, and then dried in air at 100° C. To achieve the finestpossible grain size, the powder is then ground in a stirring ball millin water until an average grain size of less than 1 μm results.

After filtration and drying, a green powder is obtained.

EXAMPLE 2

Manufacture of ytterbium-activated yttrium-aluminium-chromium mixedgarnet (Y_(2.7)Yb_(0.3)Cr₃Al₂O₁₂):

43.93 g of yttrium oxide (Y₂O₃), 14.69 g aluminium oxide (Al₂O₃), 32.86g chromium (III) oxide (Cr₂O₃), 8.52 g ytterbium oxide (Yb₂O₃) and 100 gdehydrated sodium sulphate (Na₂SO₄) are intimately mixed and heated to1100° C. in a corundum crucible for 12 hours.

After cooling, the reaction product is ground, the fluxing agent iswashed out with water, the sodium chromate produced as a side-product isreduced with sulphuric acid/iron (II) sulphate to chromium (III)sulphate, and dried in air at 100° C. To achieve the finest possiblegrain size, the powder is then ground accordingly in water in a stirringball mill.

After filtering and drying a green powder is obtained, with an averagegrain size of less than 1 μm.

EXAMPLE 3

Manufacture of praseodymium-activated yttrium-aluminium-chromium mixedgarnet (Y_(2.98)Pr_(0.02)Cr_(2.4)Al_(2.6)O₁₂):

51.39 g yttrium oxide (Y₂O₃), 20.24 g aluminium oxide (Al₂O₃), 27.86 gchromium (III) oxide (Cr₂O₃), 0.5 g praseodymium oxide (Pr₂O₃) and 100 gdehydrated sodium sulphate (Na₂SO₄) are intimately mixed and heated to1100° C. in a corundum crucible for 12 hours.

After cooling, the reaction product is ground, the fluxing agent iswashed out with water, the sodium chromate produced as a side-product isreduced with sulphuric acid/iron (II) sulphate to chromium (III)sulphate, and dried in air at 100° C. To achieve the finest possiblegrain size, the powder is then ground accordingly in water in a grindingball mill.

After filtration and drying, a light green powder is obtained, with anaverage grain size of less than 1 μm.

EXAMPLE 4

Manufacture of neodymium-activated yttrium-chromium-perovskite(Y_(0.95)Nd_(0.05)CrO₃):

55.96 yttrium oxide (Y₂O₃), 39.65 g chromium oxide (Cr₂O₃), 4.39neodymium oxide (Nd₂O₃) and 100 g dehydrated sodium sulphate (Na₂SO₄)are intimately mixed and heated to 1100° C. in a corundum crucible for20 hours.

After cooling, the reaction product is ground, the fluxing agent iswashed out with water, the sodium chromate produced as a side-product isreduced with sulphuric acid/iron (II) sulphate to chromium (III)sulphate, and dried in air at 100° C. To achieve the finest possiblegrain size, the powder is ground accordingly in water in a grinding ballmill.

After filtration and drying, a light green powder is obtained, with anaverage grain size of less than 1 μm.

The luminescent substances can, according to the invention, beintroduced to the valuable document in a variety of different ways. Forexample, the luminescent substances can be mixed into a printing ink,which additionally contains visible colour additives. Mixing of theluminescent substances into a paper clip is also possible. Likewise, theluminescent substances can be applied on or in a plastic substratematerial, which, for example, is at least partially embedded in a paperpulp. The substrate material may in this case take the form of a safetythread, a mottling thread, or a planchet.

The plastic or paper substrate material can, however, also be attachedto any other desired object, for example, for product security. In thiscase, the substrate material is preferably made in the form of a label.If the substrate material is a constituent part of the object which isto be secured, as is the case, for example, with tear-off threads, anyother shape is naturally also possible. In specific applicationinstances it may be a good idea for the luminescent substance to beprovided as an invisible coating on the valuable document. It may thenbe present over the entire surface, or in the form of specific patterns,such as strips, lines, circles, or in the form of alphanumeric symbols.

The designation “valuable document” is to be understood in the contextof the invention to mean items such as bank notes, cheques, shares,stamps, identity cards, credit cards, passes, and other documents, aswell as labels, seals, packaging, or other elements for productsecurity.

FIG. 6 shows an embodiment of a security element according to theinvention. The security element consists in this case of a label 5,which is composed of a paper or plastic layer 6, a transparent coveringlayer 7, and an adhesive layer 8. This label 5 is attached to thedesired substrate 10 by means of the adhesive layer 8. This substrate 10may be a valuable document, identity card, pass, certificate, or similaritem, or other document to be safeguard, such as CDs, packaging, etc.

The luminescent substance 9 in this embodiment is contained in thevolume of the layer 6. If this layer 6 is a paper layer, then theconcentration of luminescent substance is between 0.05 and 1% by weight.

As an alternative, the luminescent substance may also be contained in aprinting ink, not shown, which is printed on one of the layers of thelabel, preferably on the surface of the layer 6. The concentration ofluminescent substance in the printing ink varies in this case in therange between 10 and 30% by weight.

Instead of the luminescent substance being present in or on a substratematerial, which is then attached as a security element to an object, itis also possible according to the invention for the luminescentsubstance to be provided directly in the valuable document to besafeguarded, or onto its surface, in the form of a coating.

What is claimed is:
 1. Printed valuable document, with at least oneauthentication mark in the form of a luminescent substance based on ahost lattice doped with at least one rare earth metal, which largelyabsorbs light in the entire visible region of the spectrum, is excitablein substantial parts of the visible region of the spectrum and is atleast partially transparent at least in the wavelength range between 0.8and 1.1 μm, whereby the at least one rare earth metal emits in thewavelength range between 0.8 μm and 1.1 μm, and that the host latticehas a garnet or perovskite structure.
 2. Printed valuable documentaccording to claim 1, wherein the rare earth metal is ytterbium,praseodymium, or neodymium.
 3. Printed valuable document according toclaim 1, wherein the host lattice contains chromium as an absorptivecomponent.
 4. Printed valuable document according to claim 1, whereinthe garnet structure can be described by the general formulaA₃Cr_(5-x)Al_(x)O₁₂ where A stands for an element selected from thegroup consisting of scandium, yttrium, the lanthanides, and theactinides, and the index x fulfils the condition 0<x<4.99.
 5. Printedvaluable document according to claim 4, wherein the index x fulfils thecondition 0.3<x<2.5.
 6. Printed valuable document according to claim 4,wherein the luminescent substance can be described by the formulaY_(3-z)D_(z)Cr_(5-x)Al_(x)O₁₂ where D stands for an element selectedfrom the group consisting of neodymium, praseodymium, and ytterbium, andthe index z fulfils the condition 0<z<1.
 7. Printed valuable documentaccording to claim 1, wherein the perovskite structure can be describedby the general formula ACrO₃ where A stands for an element selected fromthe group consisting of yttrium, scandium, and the lanthanides. 8.Printed valuable document according to claim 7, wherein the luminescentsubstance can be described according to the formula Y_(1-z)D_(z)CrO₃where D stands for an element selected from the group consisting ofneodymium, praseodymium, and ytterbium, and the index z fulfils thecondition 0<z<1.
 9. Printed valuable document according to claim 1,wherein the luminescent substance is mixed into a printing ink, whichadditionally contains visible color additives.
 10. Printed valuabledocument according to claim 1, wherein the luminescent substance ismixed into the paper pulp.
 11. Printed valuable document according toclaim 1, wherein the luminescent substance is provided on or in asubstrate material, which is at least partially embedded in the paperpulp.
 12. Printed valuable document according to claim 11, wherein thesubstrate material consists of plastic.
 13. Printed valuable documentaccording to claim 11, wherein the substrate material is a securitythread or mottling fiber.
 14. Printed valuable document according toclaim 1, wherein the luminescent substance is provided as an invisibleand at least partial coating on the valuable document.
 15. Printedvaluable document according to claim 14, wherein the coating is astripe.
 16. Security element, which features at least one substratematerial and one luminescent substance based on a host lattice dopedwith at least one rare earth metal, which largely absorbs light in thevisible spectral region, is excitable in substantial parts of thevisible spectral region, and is at least partially transparent at leastin the wavelength range between 0.8 μm and 1.1 μm, whereby the at leastone rare earth metal emits light in the wavelength range between 0.8 μmand 1.1 I μm, and the host lattice has a garnet or perovskite structure.17. Security element according to claim 16, wherein the luminescentsubstance is provided in the volume of the substrate material. 18.Security element according to claim 17, wherein the luminescentsubstance is present in the substrate material in a concentrationbetween 0.01 and 10% by weight.
 19. Security element according to claim16, wherein the luminescent substance is present in a layer applied tothe substrate material.
 20. Security element according to claim 19,wherein the luminescent substance is present in a printing ink in aconcentration between 0.5 and 40% by weight.
 21. Security elementaccording to claim 16, wherein the substrate material consists ofplastic.
 22. Security element according to claim 16, wherein thesubstrate material consists of paper.
 23. Security element according toclaim 16, wherein the substrate material is a security thread, mottlingfiber, planchet, or label.
 24. Security element according to claim 17,wherein the luminescent substance is present in the substrate materialin a concentration between 0.1 and 5% by weight.
 25. Security elementaccording to claim 19, wherein the luminescent substance is present in aprinting ink in a concentration between 20 and 30% by weight.